As methods by which semiconductor chips are attached to lead frames, a method has been used in which a die-bonding material is fed onto the lead frame and the semiconductor chip is bonded thereto.
Such a die-bonding material is known to include, e.g., Au-Si eutectics, solders and resin pastes. Of these, Au-Si eutectics have problems in that they are expensive, have a high modulus of elasticity, and require vibration at the bonding portion. The solders have problems in that they can not withstand temperatures equal to or greater than their melting temperature and have a high modulus of elasticity.
As for the resin pastes, silver paste is the most commonly available. Compared with other materials, silver paste is inexpensive, has a high heat resistance reliability and has a low modulus of elasticity. Hence, they are most widely used as bonding materials for the lead frames of ICs and LSIs.
In recent years, there has been a rapid increase in demand for high-density packaging as electronic machinery has been made smaller in size and thickness. In semiconductor packaging, conventional pin insertion packaging has been substituted by surface packaging, which has become the prevailing packaging method suitable for high-density packaging.
In surface packaging, in order to directly solder leads to printed-wiring substrates, packaging is carried out by infrared reflowing, vapor phase reflowing or solder dipping while heating the whole package.
During this packaging, the whole package is exposed to high temperatures of 210° C. to 260° C. Hence, any presence of moisture in the package causes explosive vaporization of the moisture to cause package cracks (hereinafter “reflow cracks”).
Such reflow cracks may cause a great lowering of the reliability of semiconductor packages, bringing about a serious technical problem.
The mechanism by which reflow cracks ascribable to die-bonding materials occur is as follows: During storage of semiconductor packages, (1) die-bonding materials absorb moisture, (2) this moisture is vaporized upon heating when packaged by reflowing and soldering, and (3) vapor pressure thus produced causes breaking or separation of the die-bonding material layers, (4) so that the reflow cracks occur.
While reflow crack resistance of encapsulant has been improved, the reflow cracks ascribable to die-bonding materials provide a serious matter especially in thin-type packaging. Thus, it is strongly sought to improve its reflow crack resistance.
The silver paste, having been most commonly used, tends to cause reflow cracks because it has become difficult with the increase in size of chips to uniformly coat the silver paste on the whole surface requiring area and also because it is pasty itself and therefore tends to cause voids in bonding layers.